Trunked radio system

A central-controlled trunked system uses a control channel (as shown). Another type, Scan based trunked systems, (not shown) do not have a control channel. Frequencies are for discussion purposes and do not correspond to any specific system.

A trunked radio system is a complex type of computer-controlled two-way radio system that allows sharing of relatively few radio frequency channels among a large group of users. Instead of assigning, for example, a radio channel to one particular organization at a time, users are instead assigned to a logical grouping, a "talkgroup". When any user in that group wishes to converse with another user in the talkgroup, a vacant radio channel is found automatically by the system and the conversation takes place on that channel. Many unrelated conversations can occur on a channel, making use of the otherwise idle time between conversations. Each radio transceiver contains a microcomputer to control it. A control channel coordinates all the activity of the radios in the system. The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency.

The primary purpose of this type of system is efficiency; many people can carry many conversations over only a few distinct frequencies.[1] Trunking is used by many government entities to provide two-way communication for fire departments, police and other municipal services, who all share spectrum allocated to a city, county, or other entity.

In essence, a trunked radio system is a packet switching computer network. Users' radios send data packets to a computer, operating on a dedicated frequency — called a Control Channel — to request communication on a specific talk-group. The controller sends a digital signal to all radios monitoring that talkgroup, instructing the radios to automatically switch to the frequency indicated by the system to monitor the transmission. After the user is done speaking, the users' radios return to monitoring the control channel for additional transmissions.

This arrangement allows multiple groups of users to share a small set of actual radio frequencies without hearing each other's conversations. Trunked systems primarily conserve limited radio frequencies and also provide other advanced features to users.

A talkgroup is an assigned group on a trunked radio system. Unlike a conventional radio which assigns users a certain frequency, a trunk system takes a number of frequencies allocated to the system. Then the control channel coordinates the system so talkgroups can share these frequencies seamlessly. The purpose is to dramatically increase bandwidth. Many radios today treat talkgroups as if they were frequencies, since they behave like such. For example, on a radio scanner it is very common to be able to assign talkgroups into banks or lock them out, exactly like that of conventional frequencies.

In some agencies, groups are assigned in a fleet map. This was implemented to make it easier to assign group ID numbers. For example, EMS would be on a separate 'fleet' than police. In those fleets are subfleets or the actual talkgroups. This system is not as common as the typical trunking method, but they do exist. In some communities the ID corresponds to a location or agency. For example, it is common to see a '1' or '2' in front of a police group and another number in front of a fire group.

Most scanners that can listen to trunked radio systems (called trunk tracking) are able to scan and store individual talkgroups just as if they were frequencies. The difference in this case is that the groups are assigned to a certain bank in which the trunked system is programmed. In other words, the talkgroups are stored on the trunked bank.

The concept of trunking (resource sharing) is actually quite old, and is taken from telephone company technology and practice. Consider two telco central office exchanges, one in town "A" and the other in adjacent town "B". Each of these central offices has the theoretical capacity to handle ten thousand individual telephone numbers. (Central office "A", with prefix "123", has available 10,000 numbers from 123-0000 to 123-9999; central office "B", with prefix "124", the same.)

How many telephone lines are required to interconnect towns A & B? If all 10,000 subscribers in "A" were to simultaneously call 10,000 subscribers in "B", then 10,000 lines (in telco parlance "trunk lines", or simply "trunks") would be required between the two towns. However, the odds of that happening are remote. Telephone companies have well-proven formulas which predict the optimal number of trunk lines actually needed, under normal conditions, to interconnect two telephone exchanges, called the Erlang-B formula.

This concept has simply been applied to radio user groups, to determine the optimal number of channels needed, under normal conditions, to accommodate given number of users. In the event of a widespread emergency such as a major earthquake, many more users than normal will attempt to access both the telephone and radio systems. In both cases once the trunking capacity of the systems is fully used, all subsequent users will receive a busy signal.

In our example of police dispatch, different talk-groups are assigned different system priority levels, sometimes with "preempt" capability, attempting to ensure that communication between critical units is maintained.

"Trunked" radio systems differ from "conventional" radio systems in that a conventional radio system uses a dedicated channel (frequency) for each individual group of users, while "trunking" radio systems use a pool of channels which are available for a great many different groups of users.[2]

For example, if police communications are configured in such a way that twelve conventional channels are required to permit citywide dispatch based upon geographical patrol areas, during periods of slow dispatch activity much of that channel capacity is idle. In a trunked system, the police units in a given geographical area are not assigned a dedicated channel, but instead are members of a talk-group entitled to draw upon the common resources of a smaller pool of channels.

Trunked radio takes advantage of the probability that with any given number of user units, not everyone will need channel access at the same time, therefore fewer discrete radio channels are required. From another perspective, with a given number of radio channels, a much greater number of user groups can be accommodated. In the example of the police department, this additional capacity could then be used to assign individual talk groups to specialized investigative, traffic control, or special-events groups which might otherwise not have the benefit of individual private communications.

To the user, a trunking radio looks just like an "ordinary" radio: there is a "channel switch" for the user to select the "channel" that they want to use. In reality though, the "Channel switch" is NOT switching frequencies as in a conventional radio but when changed, it refers to an internal software program which causes a talkgroup affiliation to be transmitted on the control channel. This identifies the specific radio to the system controller as a member of a specific talkgroup, and that radio will then be included in any conversations involving that talkgroup.

This also allows great flexibility in radio usage - the same radio model can be used for many different types of system users (i.e. Police, Public Works, Animal Control, etc.) simply by changing the software programming in the radio itself.

Since the talkgroups are constantly transmitting on different frequencies, trunked radio systems makes it more difficult for a scanner listener without a programmed trunk tracking scanner to keep up with the conversation.

In 1997, radio scanners compatible with trunked systems appeared on the market. One of the first companies to bring these devices to market, Uniden, trademarked the term trunk tracking on December 5, 1997.[3]